CHAPTER X. 



GENERAL DEVELOPMENT OF THE EMBRYO. 



THERE is a close agreement in the history of the 

 development of the embryo of the various kinds of 

 Mammals. We may therefore take one, the Rabbit, as 

 a type. There are without doubt considerable varia- 

 tions to be met with in the early development even of 

 species nearly allied to the Rabbit, but at present the 

 true value of these variations is not understood, and 

 they need not concern us here. 



The ovarian ovum. Mammals possess two ovaries 

 situated in the body cavity, one on either side of the 

 vertebral column immediately posterior to the kidneys. 

 They are somewhat flattened irregularly oval bodies, a 

 portion of the surface being generally raised into pro- 

 tuberances due to projecting follicles. 



In an early stage of development the follicle in the 

 mammalian ovary is similar to that of the fowl, and is 

 formed of flat cells derived from the germinal cells ad- 

 joining the ovum. As development proceeds however 

 it becomes remarkably modified. These flat cells sur- 

 rounding the ovum become columnar and then one or 

 two layers deep. Later they become thicker on one 

 side of the ovum than on the other, and there appears 



